(a) Field of the Invention
The present invention relates to a display device and a driving method thereof, and more particularly, the present invention relates to a light emitting display device and a driving method thereof.
(b) Description of the Related Art
Recent trends of lightweight and thin personal computers and television sets also require light-weight and thin display devices, and flat panel displays satisfying such a requirement are being substituted for conventional cathode ray tubes (“CRT”).
Examples of flat panel displays include a liquid crystal display (“LCD”), field emission display (“FED”), organic light emitting diode (“OLED”) display, plasma display panel (“PDP”), etc.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and the display displays images by controlling the luminance of the pixels based on given luminance information. An OLED display is a self-emissive display device that displays images by electrically exciting light emitting organic material, and the OLED display has low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED display includes an OLED and a driving thin film transistor (“TFT”). The OLED emits light having an intensity depending on the current driven by the driving TFT, which in turn depends on the threshold voltage of the driving TFT and the voltage between gate and source of the driving TFT.
The TFT includes polysilicon or amorphous silicon a-Si. A polysilicon TFT has several advantages, however it also has disadvantages such as the complexity of manufacturing polysilicon, thereby increasing the manufacturing cost. In addition, it is hard to make a large OLED display employing polysilicon TFTs.
On the contrary, an a-Si TFT is easily applicable to a large OLED display and is manufactured by a lesser number of process steps than the polysilicon TFT. However, the threshold voltage of the a-Si TFT shifts as time passes under a long-time application of a DC control voltage such that the luminance is varied for a given data voltage.
Also, a long time driving of the OLED shifts the threshold voltage of the OLED. As for an OLED display employing an n-type driving TFT, since the OLED is connected to the source of the driving TFT, the shift of the threshold voltage of the OLED changes the voltage at the source of the driving TFT to vary the current driven by the driving TFT. Accordingly, the image quality of the OLED display may be degraded.
The shift of the threshold voltage of the driving transistor and the OLED may be compensated by providing several transistors between a driving voltage and the OLED. However, the several transistors may consume large power.